Method of making an iron/polymer powder composition

ABSTRACT

A powder composition of iron-based powder particles to which is bonded a polymeric material is prepared by making a dry admixture of the iron-based particles and particles of the polymer, wetting the admixture with a solvent for the polymeric material, and removing the solvent to leave a flowable powder composition. The powder compositions can be compression molded, generally at a temperature above the glass transition temperature of the polymer, to form magnetic core components.

FIELD OF THE INVENTION

This invention relates to a method of making a powder compositioncomprising iron-based powder particles to which is bonded polymericmaterial, generally in particulate form. More specifically, the methodof the invention is directed to the use of a solvent for the polymericmaterial to effect bonding of the polymer to the iron particles afterthe iron and polymer are first admixed in dry form. The powdercompositions so made are particularly useful to make magnetic corecomponents.

BACKGROUND OF THE INVENTION

Iron-based particles have long been used as a base material in themanufacture of structural components by powder metallurgical methods.The iron-based particles are first molded in a die under high pressuresin order to produce the desired shape. After the molding step, thestructural component usually undergoes a sintering step to impart thenecessary strength to the component.

Magnetic core components have also been manufactured by such powermetallurgical methods, but the iron-based particles used in thesemethods are generally coated with a circumferential layer of insulatingmaterial.

Two important characteristics of an iron core component are its magneticpermeability and core loss characteristics. The magnetic permeability ofa material is an indication of its ability to become magnetized, or itsability to carry a magnetic flux. Permeability is defined as the ratioof the induced magnetic flux to the magnetizing force or fieldintensity. When a magnetic material is exposed to a rapidly varyingfield, the total energy of the core is reduced by the occurrence ofhysteresis losses and/or eddy current losses. The hysteresis loss isbrought about by the necessary expenditure of energy to overcome theretained magnetic forces within the iron core component. The eddycurrent loss is brought about by the production of electric currents inthe iron core component due to the changing flux caused by alternatingcurrent conditions.

Early magnetic core components were made from laminated sheet steel, butthese components were difficult to manufacture and experienced largecore losses at higher frequencies. Application of these lamination-basedcores is also limited by the necessity to carry magnetic flux only inthe plane of the sheet in order to avoid excessive eddy current losses.Sintered metal powders have been used to replace the laminated steel asthe material for the magnetic core component, but these sintered partsalso have high core losses and are restricted primarily to directcurrent operations.

Research in the powder metallurgical manufacture of magnetic corecomponents using coated iron-based powders has been directed to thedevelopment of iron powder compositions that enhance certain physicaland magnetic properties without detrimentally affecting otherproperties. Desired properties include a high permeability through anextended frequency range, high pressed strength, low core losses, andsuitability for compression molding techniques.

When molding a core component for AC power applications, it is generallyrequired that the iron particles have an electrically insulating coatingto decrease core losses. The use of a plastic coating over the ironparticles (see U.S. Pat. No. 3,935,340 to Yamaguchi) and the use ofdoubly-coated iron particles (see U.S. Pat. No. 4,601,765 to Soileau etal.) have been employed to insulate the iron particles and thereforereduce eddy current losses.

Recently, it has been found that the insulating polymeric material neednot be present in the powder composition as a full coating of theindividual iron particles, but rather can be present in the form ofdiscrete particles that are integrally admixed with the iron particles.The present invention is directed to a method of forming this admixturein a manner that ensures homogeneity and thereby leads to improvedmagnetic properties of pressed parts made with the powder composition.The invention eliminates the need to provide the iron-based particleswith a circumferential coating of the polymeric material, which coatinggenerally required the use of more expensive fluidized bed processes.

SUMMARY OF THE INVENTION

The present invention provides a method of making an iron/polymer powdercomposition that comprises iron-based powder particles and a polymericmaterial that is bonded to the iron-based particles. According to themethod, a dry admixture of the iron-based particles and particles of thepolymer is made. Generally, the polymeric material will constitute about0.001-15.0% by weight of the combined weights of the iron-basedparticles and polymer. The dry admixture is then wetted with a solventfor the polymeric material. Preferably, the solvent is sprayed onto theadmixture while the admixture is being further mixed. Thereafter, thesolvent is removed, leaving a flowable powder composition of theiron-based particles to which is bonded the polymeric material.

The method is applicable to forming a powder composition from anyiron-based particles and polymeric material. By "iron-based" is meantany of the iron-containing particles generally used in powdermetallurgical methods including, but not limited to, particles ofsubstantially pure iron, and particles of iron that has been pre-alloyedwith, for example, transition metals and/or other fortifying elements.Although the method is applicable to use of any polymeric material, ithas its most significant utility with respect to the use ofthermoplastic materials that are good insulators for magnetic corecomponents. Preferred thermoplastic materials are biphenylene ethers andpolyether imides, and the preferred solvent for use with these polymersis methylene chloride.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, it has been found that polymericinsulating materials can be glued or bonded to iron-based powders toprovide a composition that has substantial homogeneity and that willprovide, upon compaction, magnetic core components having excellentmagnetic properties. According to the present invention, the iron-basedparticles and particles of the polymer material are first admixed,generally in a dry state, to form a substantially homogeneous admixtureof the two. The dry admixture is then contacted by a relatively smallamount of a solvent for the polymer in a manner, as more fully describedbelow, that ensures substantial wetting of the entire admixture.Thereafter, the solvent is removed, providing a dry, flowable powdercomposition of iron-based particles to which is bonded the polymericmaterial. Magnetic core components can then be produced by the powdercompositions by known methods.

The iron-based particles that are useful in the invention are any of thepure iron or iron-containing (including steel or ferromagnetic)particles generally used in powder metallurgical methods. Examples areparticles of substantially pure iron and particles of iron pre-alloyedwith other elements (for example, steel-producing elements) that enhancethe strength, hardenability, electromagnetic properties, or other.desirable properties of the final product. The particles of iron-basedmaterial useful in this invention can have a weight average particlesize as small as one micron or below, or up to about 850-1,000 microns,but generally the particles will have a weight average particle size inthe range of about 10-500 microns. Preferred are particle compositionshaving a maximum average particles size of about 350 microns, and morepreferred are particle compositions having a maximum average particlesize of about 250 microns.

The preferred iron-based particles for use in the invention are highlycompressible powders of substantially pure iron; that is, ironcontaining not more than about 1.0% by weight, preferably no more thanabout 0.5% by weight, of normal impurities. Examples of suchmetallurgical grade pure iron powders are the ANCORSTEEL 1000 series ofiron powders available from Hoeganaes Corporation, Riverton, N.J. Aparticularly preferred such powder is ANCORSTEEL 1000C iron powder,which has a typical screen profile of about 13% by weight of theparticles below a No. 325 sieve and about 17% by weight of the particleslarger than a No. 100 sieve with the remainder between these two sizes(trace amounts larger than No. 60 sieve). The ANCORSTEEL 1000C powderhas an apparent density of from about 2.8 to about 3.0 g/cm².

Other iron-based powders that are useful in the practice of theinvention are ferromagnetic or steel powders containing effectiveamounts of alloying elements pre-alloyed with the iron. Examples of goodferromagnetic materials are particles of iron pre-alloyed with smallamounts of phosphorus. Other good ferromagnetic materials are blends offerrophosphorus powders, such as iron-phosphorus alloys or ironphosphide compounds in powdered form, admixed with particles ofsubstantially pure iron. Such powder mixtures are disclosed in U.S. Pat.No. 3,836,355 issued September 1974 to Tengzelius et al. and U.S. Pat.No. 4,093,449 1978 to Svensson et al. Examples of steel powders areparticles of iron pre-alloyed with one or more transition elements orother fortifying elements, such as molybdenum, nickel, manganese,copper, and chromium. Various pre-alloyed steel powders that can be usedin the practice of this invention are available from Hoeganaes Corp. aspart of its ANCORSTEEL line of steel powders.

The iron-based particles can first be coated with an insulativeinorganic material to provide an inner coating that underlies the bondedpolymeric material. This inner coating is preferably no greater thanabout 0.2% by total weight of the coated particle. Such inner coatingsinclude iron phosphate, such as disclosed in U.S. Pat. No.5,063,011issued November 1991 to Rutz et al, and alkaline metal silicates, suchas disclosed in U.S. Pat. No.4,601,765 issued July 1986 to Soileau etal. The disclosures of these patents are hereby incorporated byreference.

Any polymeric material that can be sufficiently softened and/ordissolved by a solvent so as to be able to adhere or bond to thesurfaces of the iron-based particles can be used in this invention.Preferred polymeric materials are thermoplastic materials, particularlythose that have a weight average molecular weight in the range of about10,000 to 50,000. More preferred are thermoplastic polymers of such amolecular weight range that have a glass transition temperature in therange of about 175°-450° F. (about 80°-230° C.). Examples of thethermoplastic material are polyetherimides, polyphenylene ethers,polyethersulfones, polycarbonates, polyethylene glycol, polyvinylacetate, and polyvinyl alcohol.

Suitable polycarbonates that can be utilized as a thermoplastic in thepresent invention are bisphenol-A-polycarbonates, also known aspoly(bisphenol-A-carbonate). These polycarbonates have a specificgravity range of about 1.2 to 1.6. A specific example ispoly(oxycarbonyloxy-1,4-phenylene-(1-methylethlidene)-1,4-phenylene)having an empirical formula of (C₁₆ H₁₄ O₃)_(n) where n is an integer ofabout 30-60. Commercially available polycarbonates are the LEXAN resinsfrom General Electric Company. The most preferred LEXAN resins are theLEXAN 121 and 141 grades.

A suitable polyphenylene ether thermoplastic ispoly(2,6-dimethyl-1,4-phenylene oxide) which has an empirical formula of(C₈ H₈ O)_(n) where n is an integer of about 30-100. The polyphenyleneether homopolymer can be admixed with an alloying/blending resin such asa high impact polystyrene, such as poly(butadiene-styrene); and apolyamide, such as Nylon 66 either a s polycaprolactam orpoly(hexamethylenediamine-adipate). These thermoplastic materials have aspecific gravity in the range of about 1.0 to 1.4. A commerciallyavailable polyphenylene is sold as NORYL resin by the General ElectricCompany. The most preferred NORYL resins are the NORYL 844, 888, and1222 grades.

A suitable polyetherimide thermoplastic ispoly[2,2'-bis(3,4-dicarboxyphenoxy) phenylpropane)-2-phenylene bismide]which has an empirical formula of (C₃₇ H₂₄ O₆ N₂)_(n) where n is aninteger of about 15-27. The polyetherimide thermoplastics have aspecific gravity in the range of about 1.2 to 1.6. A commerciallyavailable polyetherimide is sold as ULTEM resin by the General ElectricCompany. The most preferred ULTEM resin is the ULTEM 1000 grade.

A suitable polyethersulfone thermoplastic has the general empiricalformula of (C₁₂ H₁₆ SO₃)_(n) where n is an integer of about 50-200. Anexample of a suitable polyethersulfone which is commercially availableis sold as VICTREX PES by ICI, Inc The most preferred of these resins isthe VICTREX PES 5200 grade.

For use in the invention, the polymeric material is generally providedin the form of particles, which will preferably be spherical but can be,for example, lenticular or flake-shaped. The particles are preferablyfine enough to pass through a No. 60 sieve, U.S. Series (about 250microns or less), more preferably through a No. 100 sieve (about 150microns or less) and most preferably through a No. 140 sieve (about 105microns or less). The absolute size of the polymer particles is lessimportant, however, than their size in relation to the size of theiron-based particles; it is preferred that the polymer particlesgenerally be finer than the iron-based particles. The amount of polymeris generally about 0.001-15% by weight of the total weight of theiron-based particles and polymeric particles. Preferably the polymer isat least about 0.2% by weight, up to about 5% by weight, of thiscombination. More preferably the polymer is about 0.4-2% by weight, andmost preferably about 0.6-1.0% by weight, of the combined weight of theiron-based particles and polymer material.

The iron-based particles and polymeric particles are admixed together,preferably in dry form, by conventional mixing techniques to form asubstantially homogeneous particle blend. The dry admixture is thencontacted with sufficient solvent to wet the particles, and moreparticularly to soften and/or partially dissolve the surfaces of thepolymeric particles, causing those particles to become tacky and toadhere or bond to the surfaces of the iron-based particles. Preferablythe solvent is applied to the dry admixture by spraying fine droplets ofthe solvent during mixing of the dry blend. Most preferably mixing iscontinued throughout the solvent application to ensure wetting of thepolymer materials and homogeneity of the final mixture. The solvent isthereafter removed by evaporation, optionally with the aid of heating,forced ventilation, or vacuum. Mixing can be continued during thesolvent removal step, which will itself aid evaporation of the solvent.The initial dry blending of the particles as well as the application andremoval of the solvent can be effected in conventional mixing equipmentoutfitted with suitable solvent application and recovery means. Theconical screw mixers available from the Nauta Company can be used forthis purpose.

Any organic solvent for the polymeric material can be used. Preferredare methylene chloride, 1,1,2-trichloroethane, and acetone. Blends ofthese solvents can also be used. A preferred combination for use in thisinvention uses a polyetherimide thermoplastic as the polymeric materialand methylene chloride as the solvent. The amount of solvent applied tothe dry admixture will be about 1-25 weight parts solvent per 100 weightparts of iron-based powder. Generally, however, it is more convenient tocalculate the amount of solvent based on the amount of polymericmaterial present. In these terms, about 1.5-50 weight parts, preferablyabout 3-20 weight parts, more preferably about 5-10 weight parts ofsolvent per unit weight part of polymer, will sufficiently wet theadmixture.

The iron/polymer powder compositions made by the method of thisinvention can be formed into magnetic cores by an appropriate moldingtechnique. In preferred embodiments, a compression molding technique, inwhich the powder composition is charged into a die and heated to atemperature above the glass transition temperature of the thermoplasticmaterial, is used to form the magnetic components. Preferably, the dieand composition are heated to a temperature that is about 25-85Centigrade degrees above the glass transition temperature. Normal powdermetallurgy pressures are applied at the indicated temperatures to pressout the desired component. Typical compression molding techniques employcompaction pressures of about 5-100 tons per square inch (69-1379 MPa),preferably in the range of about 30-60 tsi (414-828 MPa). A lubricant,usually in an amount up to about 1% by weight, can be mixed into theiron/polymer powder composition, although the lubricant can be applieddirectly on the die wall. Use of the lubricant reduces stripping andsliding pressures. Examples of suitable lubricants are zinc stearate orone of the synthetic waxes available from Glycol Chemical Co. as ACRAWAXsynthetic wax. Another lubricant that can be admixed directly with theiron/polymer powder composition is particulate boron nitride.

Following the compaction step, the molded component is optionally heattreated. According to this procedure, the molded component, preferablyafter removal from the die and after being permitted to cool to atemperature at least as low as the glass transition temperature of thepolymeric material, is separately heated to a "process" temperature thatis above the glass transition temperature, preferably to a temperatureup to about 140 Centigrade degrees above the temperature at which thecomponent was compacted. The molded component is maintained at theprocess temperature for a time sufficient for the component to bethoroughly heated and its internal temperature brought substantially tothe process temperature. Generally, heating is required for about 0.5-3hours, depending on the size and initial temperature of the pressedpart. The heat treatment can be conducted in air or in an inertatmosphere such as nitrogen.

EXAMPLE 1

An iron/thermoplastic powder composition was prepared using particulateULTEM 1000 polyetherimide (screened to exclude particles larger than 150microns) and particles of substantially pure iron that had been annealedin dissociated ammonia and screened to exclude particles smaller than150 microns and larger than about 375 microns. The iron particles andthermoplastic particles were hand mixed in a dry state in amountsproviding an iron/thermoplastic composition that was 0.6% by totalweight of the thermoplastic material. Test compositions were prepared byspraying methylene chloride onto the dry admixture, in the amountsspecified below, while mixing was continued. Mixing of the wettedadmixture was continued for an additional 2-3 minutes after the solventaddition was completed to aid in attaining a homogeneous mixture and toaid in the removal of the solvent by evaporation. At the end of thisperiod, the admixture was spread out on a tray and allowed to dry inair.

Three different test compositions were prepared by the above method,using varying amounts of methylene chloride, as follows: Composition Awas made using 1.7 weight parts of solvent per 100 weight parts of ironpowder; Composition B was made using 3.3 weight parts solvent per 100weight parts iron powder; and Composition C made using 8.3 weight partssolvent per 100 weight parts of iron powder. A dry admixture, which hadnot been treated with the solvent bonding technique according to thepresent invention, was also retained as a control composition.

Toroids and strength-test bars were prepared from the powdercompositions in order to determine magnetic properties and transverserupture strength. The material was uniformly molded in a die at atemperature of 525° F. (about 274° C.) under a pressure of 40 tsi (about552 MPa). Following compaction, the pressed pieces were heat-treated ata temperature of 600° F. (about 316° C.) for one hour in air. Transverserupture strength was determined for the heat-treated test bars accordingto ASTM B528-76. Results are tabulated in Table 1A. As can be seen, thestrength of the parts increased at the higher solvent levels.

                  TABLE 1A                                                        ______________________________________                                        Transverse Rupture Strength (psi)                                                       Control  A          B     C                                         ______________________________________                                        Heat-Treated                                                                            26,700   21,900     28,800                                                                              34,300                                    ______________________________________                                    

AC core loss (watts/pound; measured at 500 Hz, 1 Tesla), was determinedfor the heat-treated toroids. Results are shown in Table 1B below. Thetoroid made from the control (no solvent bonding) composition exhibitedthe highest core loss, indicating that use of the solvent providedimprovement over the simple admixture.

                  TABLE 1B                                                        ______________________________________                                                      Control  A         B   C                                        ______________________________________                                        AC Core Loss  439      82        56  55                                       ______________________________________                                    

EXAMPLE 2

An iron/thermoplastic powder composition was prepared using particulateULTEM 1000 polyetherimide (screened to exclude particles larger than 104microns) and ANCORSTEEL 1000C iron powder. The iron particles (297.75pounds) and thermoplastic particles (2.25 pounds) were mixed in a Nautaconical screw mixer for 15 minutes. Methylene chloride (15 pounds) wasthen added to the mixture over a period of 15 minutes while the mixingcontinued. Mixing of the wetted admixture continued for an additional 10minutes. The solvent was thereafter removed by subjecting the mixingvessel to a vacuum while the mixing continued. The resultant dry powderwas then discharged from the mixing vessel.

Strength-test bars and toroids were prepared and tested as described inExample 1. The results were compared to the same control sample used inExample 1. Results are tabulated in Table 2.

                  TABLE 2                                                         ______________________________________                                        AC Core Loss and Transverse Rupture Strength                                                   Control  Test Sample                                         ______________________________________                                        Core Loss 500 Hz, 1                                                                              439       45                                               Testa (Watts/pound)                                                           Transverse Rupture                                                                             26,700   38,320                                              Strength (psi)                                                                ______________________________________                                    

What is claimed is:
 1. A method of making an iron/polymer powdercomposition of iron-based powder particles to which is bonded apolymeric material, the method comprising:(a) forming a dry admixture ofiron-based powder particles and particles of a polymeric material, thepolymer constituting about 0.001-5.0% by weight of the admixture; (b)wetting the dry admixture with a solvent for the polymeric material; and(c) removing the solvent.
 2. The method of claim 1 wherein material is athermoplastic polymer and the iron-based powder particles aresubstantially pure iron.
 3. The method of claim 1 wherein the polymericmaterial is a thermoplastic material having a weight average molecularweight of about 10,000-50,000.
 4. The method of claim 2 wherein thethermoplastic material has a glass transition temperature in the rangeof about 80°-230° C. and constitutes about 0.2-5.0% by weight of theadmixture.
 5. The method of claim 2 wherein the thermoplastic materialis a polyetherimide, a polyphenylene ether, a polyethersulfone, or apolycarbonate.
 6. The method of claim 1 wherein said wetting stepcomprises spraying the solvent onto the admixture while further mixingthe admixture, wherein the iron particles have a weight average particlesize up to about 500 microns and wherein the particles of polymericmaterial are finer than about 250 microns.
 7. The method of claim 5wherein said wetting step comprises spraying the solvent onto theadmixture while further mixing the admixture, and wherein the solvent ismethylene chloride.
 8. The method of claim 5 wherein the iron particleshave a weight average particle size up to about 500 microns and whereinthe particles of polymeric material are finer than about 250 microns. 9.The method of claim 5 wherein the polymer constitutes about 0.4-2% byweight of said dry admixture.
 10. The method of claim 8 wherein thepolymer constitutes about 0.4-2% by weight of said dry admixture.
 11. Amethod of making an iron/polymer powder composition of iron-based powderparticles to which is bonded a polymeric material, the methodcomprising:(a) forming a dry admixture of iron-based powder particlesand particles of a polymeric material, the polymer constituting about0.6-1% by weight of the admixture, wherein the iron-based particles havea weight average size up to about 500 microns, and wherein the particlesof polymeric material are finer than about 150 microns; (b) wetting thedry admixture with a solvent for the polymeric material; and (c)removing the solvent.
 12. The method of claim 11 wherein thethermoplastic material comprises a polyetherimide, a polyphenyleneether, a polyethersulfone, or a polycarbonate, wherein the particles ofthermoplastic are finer than about 105 microns, and wherein theiron-based particles have a weight average size less than about 250microns.
 13. The method of claim 12 wherein the solvent comprisesmethylene chloride.